coalflot.doc

Mineral Processing – lab rcise Coal flotation 1. Flotation water hydrophilic particle gas bubble intergrowths hydrophobic particle Flotation is one of the many s of separation. It could be used for separation of phases for instance to remove solid particles or oil drops from water. More frequently flotation is used for separation of particles having different hydrophobicities. Hydrophobicity is a feature of material characterizing its ability to be wetted with a liquid in the presence of a gas phase. Solids, which can be easily wetted with water, are called hydrophilic while solids with limited affinity for wetting are called hydrophobic. As a result of hydrophobicity particles adhere to the gas bubble ing a particle-air aggregate which is lighter than water, and travels upwards to the surface of water Fig. 1.. The hydrophilic particles do not adhere to the bubbles and fall down to the bottom of the flotation tank. Fig. 1. Flotation The hydrophilic-hydrophobic character of materials result from their physicochemical properties or more precisely from a balance of forces operating at the three solid-water, water-gas and solid-gas interfaces. These forces make the bubble to assume the shape and angle with the solid surface leading to minimization of the total energy of the system. That angle is called the contact angle q Fig.2. woda q pęcherzyk ziarno Fig. 2. Contact angle and it is govern by the Young equation gsg gsl glg cos q 1 where gsg - solid – gas interfacial tension in mN/m or mJ/m2 gsl - solid – liquid interfacial tension glg - liquid – gas interfacial tension q - contact angle in degrees. A thermodynamic analysis of the flotation system indicates that the main parameter of flotation is a combination of contact angle and surface tension DGf glg cos q - 12 where DGf – Gibbs thermodynamic potential free enthalpy of flotation, mJ/m2. When the surface tension of a given flotation system is constant, the contact angle becomes the main parameter of flotation as well as a measure of hydrophobicity of materials. The contact angle is measured between the lines drawn from the point when the all three phase meet Fig.2. It is a common practice to measure the angle of contact between the liquid-gas and solid-liquid interfaces through the liquid phase. Hydrophobicity of selected materials is given in Table 1. Table 1..Hydrohobicity of materials. Contact angle is in degrees and is based on flotometric measurements Strongly hydrophobic* hydrophobic weakly hydrophobic hydrophilic** q 0 Material q Material q Material q Material 1 2 3 4 5 6 7 Paraffin CnH2n2 90 sulfides 44–0 fluorite, CaF2 10–13 gypsum CaSO42H2O Teflon, C2F4 90 silicon carbide SiC 27,6 arsenic, As2O3 9,3 ironsilicon Sulfur, S 63,2 coal 26–0 perovskite, CaTiO3 9 dolomite CaMgCO32 Merkury, Hg 45,6 indum, In 25 szelit, CaWO4 9 magnetite Fe3O4 Ge 39,7 jodargyryt, AgI 23,5 diament, C 7,9 halite, NaCl Si 35,4 cassiterite, SnO2 22– tin, Sn 7,5 brawn coal Talc 35,2 silver, Ag 14 boric acid, H3BO3 6,4 kaolinite ilmenite, Fe 14 graphite, C 6,2 hematite, Fe2O3 molibdenite, MoS2 5,9 PbJ2 6 quartz, SiO2 gold, Au 5 calcite, CaCO3 barite, BaSO4 5 anhydrite, CaSO4 corundum, Al2O3 4 bones HgO 3,3 turmaline HgJ2 3 vegetables copper, Cu 3 iron, Fe amber ice, D2O *Flotometric is able to measure contact angles smaller than 90o. **Other hydrophilic materials chromite, malachite, smitsonite, azurite, rutile, zircon, mica. It results from Table 1 that most solid materials are only slightly hydrophobic or hydrophilic. For these materials to float an application of a collector is necessary. Typical collectors used for rendering certain groups of minerals hydrophobic are given in Table 2. Table 2. Classification of minerals according to their flotation properties Class Example Applied collectors Non-metals and solids with significant natural hydrophobicity sulfur, graphite, coal, talc hydrocarbons, nonionic liquids insoluble in water Native metals and sulfides gold, chalcocite chalcopyrite, galena sfalerite xanthates, aerofloats Oxidized minerals of non-ferrous metals cerusyte, smitsonite malachite, tenorite, cuprite xanthates after sulfidization siarczkowaniu, anionic and cationic Oxides, hydroxides and silicates hematite, ilmenite corundum, cassiterite chromite, feldspars kaolinite anionic and cationic with and without activation using metal ions Sparingly soluble salts fluorite, barite, calcite, apatite, dolomite anionic and cationic Soluble salts halite, silvinite, carnalite kiserite cationic, seldom anionic For successful froth flotation we need not only collector but also a frother. The role of frother is to keep the floating particles in the most upper layer called froth for easy removal of floating particles from the flotation system. The reagents used as frothers are presented in Table 3. Table 3. Flotation frothers Group Frother 1. aliphatic alcohols a linear from amyl to decanol b branched iso-amyl methyloisobutylocarbinol c with additional group diacetone 2. Cyclic a linear cyclohexanol b branched terpineol 3. Aromatic cresols xylenols 4. Alkoxy-hydrocarbons 1,1,3-trietoxybuthane 5. Polyglycols RXnOH RH lub CnH2n1 XEO ethylene oxide, PO prophylene oxide BO buthylene oxide from 3 to 7 The structure of froth is presented in Fig. 3 Fig. 3. Schematic presentation of froth and its structure with changes with the position height in the flotation cell. Not to scale magnification of upper layers is greater Third group of reagents used in flotation is called the modifying reagents. The group can be further divided into activators, depressant, pH, Eh, aggregation regulators, etc. 2. Coal Coal is an organic material of complex nature and structure. It does not have a specific chemical ula. It contains many functional groups and units, mostly aromatic, then aliphatic, and some oxygen while other elements such as S and N are in smaller amounts. A very simplified ula of coal is given in Fig. 4. Fig. 4. Simplified coal structure indicating typical chemical groups present in the coal. Oxygen present in coal can exist as CO, -COOH, -O-CH3, COC groups. Coalification of coal is a process leading to an increase of carbon content in the carbonaceous matter. The final stage of coalification is the ation of graphite Fig. 5. Fig. 5. Coalification leads to increase in carbon content. The final product is graphite The hydrophobicity of coal depends on its coalification degreeFig. 6. Fig. 6. Influence of coal coalification on contact angle sessile drop The contact angle of coal is greatly influenced by the of measurement Fig. 7. Fig.7. Contact angle of coal depends on of measurement qr – receding contact angle Coal is subjected to flotation to separate carbonaceous combustible matter from ash ing minerals. Usually only fine coal particles size below 0.5 mm is used for flotation while greater particle are processed by other separation techniques, mostly gravity s. Coal contains from few to several percent of ash. Apolar oils mostly fuel oil are used for flotation of coal. Flotation is possible due to spreading of apolar oils on non-polar sites of coal. The adsorption is physical in nature and is due to the van der Waals forces. Adsorption increases with hydrophobic of coal. Any reagent mentioned in Table 3 can be used in flotation of coal as frother. In most cases, the frother is a mixture of reagents, frequently waste products. Their commercial names and composition are subject to changes and modifications. The main components of the frother mixture are higher aliphatic alcohols and alkyl polyethoxy ethers. Some coals are difficult to float. They need additional special reagent called promoters usually special alkyl polyethoxy ethers. Flotation of coal increases in the presence of salts. The salts as a rule increase the kinetics of flotation. A special addition of salt for coal flotation is not practiced due to corrosion problems. EXPERIMENTAL PART Flotation of coal in Denver or Mechanobr laboratory flotation machine 1. Preparation of materials, reagents and equipment a Preparation of coal Use finely ground coal, which was previously ground by technicians in stages in different grinding machines. Weigh 300 gram of the coal with a technical balance with one decimal point accuracy. b Preparation of porcelain crucibles. The crucibles should be clean, dry, and stored in a desiccator to keep their mass constant. They should be marked with a number on the outside of the bottom. If they are not in desiccator, the mass of the crucible can be slightly greater than real one due to moisture adsorption. The mass of the crucible should be known with a 4-decimal-points accuracy. c Preparation of flotation collector Fuel oil, dispersed in water, will be used as the collector hydrophobization agent. You will need 300 mg of fuel oil per 1kg of impure coal. Calculate how much oil in cm3 is needed for flotation of 300 g of coal knowing that the density of fuel oil is 0.9g/cm3. Insert the oil with a pipette into a 400 cm3 beaker containing 100 cm3 of water. Use ultrasonic treatment for about 30 seconds to prepare the emulsion. Prepare emulsion shortly before use in flotation. Otherwise the emulsion will separate into oil on top of water surface. c Preparation of frother Use a- terpineol as a frother. You will need to use 150 mg/kg of the frother. The stock solution of a- terpineol is 0.1 solution. Calculate how many cubic centimeters of the 0.1 solution of a- terpineol aqueous solution you will need for flotation. Measure that value with a small graduated cylinder and get it ready. d Preparation of flotation machine The laboratory flotation machines should be ready for experiment. Make sure that you use 1.5 dcm3 metal flotation cell for flotation in Denver and 1.0 dcm3 plastic cell for Mechanobr machine. Read the safety rules for operating the machine. Prepare 6 glass containers 5 having at least 0.5 dcm3 in volume for collecting the flotation products and one larger 2 dm3 for the tailing. 2. Running the fractionating flotation test Fill the flotation cell with 1.2 dm3 of water. Start the flotation machine with the air valve closed and add the coal. Stir the flotation system for 5 minutes for wetting the coal. Next, add freshly well-dispersed fuel oil emulsion and stir the system for additional 3 minutes. Finally add the frother aqueous solution and stir the pulp for 1 min. After that slowly open the air valve and start to collect the first concentrate in the first glass container or a pan. Make sure that you collect not too much of the concentrate about 30-50 g. Collect second concentrate of coal into a subsequent container for such time intervals so each one will contain approximately similar amounts of the solids. Collect no less than 4 products. When the flotation froth become empty no solids can be filled when the froth is tested with fingers stop the flotation machine. The remaining material in the cell is the tailing of flotation. 3.uation of flotation perance a Determination of the yield of flotation products Remove water from each flotation product by filtration in a Buchner funnel under vacuum with a water pump. The moist filter cake of all products should be dried in an oven at 105C. To speed up the uation of yield you may weigh the moist products with a technical balance and then, assuming identical moisture of each sample calculate the dried mass of the samples. Take a few gram-sample from each moist product, spread it on a bottom of a flat glass container and put them into oven at 105o C for 5 minutes to dry the sample. Take about 0.5-g sample and weigh it very precisely with an electronic balance with 4 decimal points accuracy. Put the samples to pre-weighed crucible. b Determination of the ash and carbonaceous matter content in the flotation product. Dry coal consists of carbonaceous matter and ash ing minerals. Burning coal leaves ash in the container. The ash content in coal should be determined applying appropriate standard procedures. The American procedure ASTM D 3174 requires that about 1 g coal to be put into a porcelain crucible and introduced to a room-temperature furnace and heat up to 450-500o C within 1 hour and within two hours to 700-750o C, and kept at this temperature for 2 hours. Next the furnace is turned off and allowed to cool down. Burning should be pered in the presence of sufficient amount of air. Calculate the content of ash in each product of flotation as an average value of two ash determinations. c uation of flotation perance Having the yield of flotation products and their quality ash carbonaceous matter content100 calculate cumulative yields, cumulative contents, and cumulative recoveries using a balance sheet discussed in the mineral processing class. Draw three different upgrading curves, for instance the Mayer cumulative recovery versus cumulative yield, Hall cum. recovery versus cum. content, and Fuerstenau cum recovery versus cum. recovery upgrading curves. Plot also the no-separation and ideal separation lines in the graphs. uate the degree of separation basing on these lines and your upgrading curve. Predict values of separation parameters for the case when recovery of the carbonaceous matter is 90. 10